Adding UV Mapping to Mesh
Blender has a difficult texture mapping system.

To view an image on a 3D model, you need to do the following:


Select a mesh object and go to Edit Mode.
If the mesh has not been unwrapped, define your seams (Ctrl + E > Mark Seams) and unwrap it (U). Only the selected faces will be unwrapped, so make sure you have what you want unwrapped selected. Also, only the Unwrap choice guarantees that faces will have the same more or less spacial layout in UV space as they have in the 3D mesh surface, seams included.


Switch to a UV Image Editor while still in Edit Mode.
If your mesh has been unwrapped and there are some faces selected in the 3D Viewport, these faces' uvs should appear in the UV Image Editor. If this does not happen, make sure there is no render result displayed on the UV Image Editor.
Now you can assign an image to your mesh by either opening a new one from the menu Image > Open, selecting an already opened image from the drop down menu or generating a new image from the menu Image > New.
This image will be applied to the selected faces. However, it may not appear in the 3D Viewport yet. For that to happen you have to set your 3D Viewport to Textured View by hovering the mouse inside it and pressing Alt + Z. You can also change it the view manually in the footer.


If you try to render your mesh, there will be no image on it. The image has simply been attached to the mesh's UV layer. To make the mesh render with this image, you need to explicitly create a material, add a texture and change the texture's type to Image, set the image, change the Mapping to UV and assign the texture to color.






Yes, it is long winded and confusing. Sorry :)